Intake and Exhaust Size - Enginology

This month, we'll step out of the "theoretical box" and into one that's a bit more practical by discussing and focusing on some fundamental ways of relating inlet and exhaust path sizing to an engine's ability to produce torque. In the process, it's important to understand that what follows can be used as a diagnostic approach to determining why torque boosts occurred where they did, as well as a tool for helping shape a given torque curve to match where an engine needs to perform best.

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We'll lace all this with a few examples, just to reinforce the points shared. Also, keep in mind that the approach we'll take is pursued somewhat at the risk of oversimplification because there are far more precise and encompassing methods available. What follows are the type of tools that work well in the dyno room or for basic parts selection, void of any intricate math or computer programs.

We've previously mentioned that both intake and exhaust flow is unsteady and somewhat pulsating, punctuated by interruptions that include the opening and closing of valves and pressure excursions involving the combustion space.

The study of wave motion plays into this, among other analyses. But for purposes of our discussion, let's say there will be a "mean flow velocity" in intake and exhaust paths that occur at or very near peak volumetric efficiency (a torque peak) in both these paths. A commonly-accepted value for this is 240 feet/second. Although many intake manifolds have runners with taper (or slightly varying cross section areas) and some headers incorporate "steps" or sudden changes in flow path cross section, we'll initially assume constant flow path areas and then examine the non-constant areas later in our discussion.

Suppose we begin by evaluating how what we've called "mean flow velocity" plays into intake manifold function. Reliable information has shown an engine's torque peak is directly linked to a mean flow velocity of 240 feet/second. Since flow path section area, engine speed, and piston displacement dictate where in the rpm range this flow rate is reached, there is a mathematical relationship from which any one of these can be determined, if the other values are known or assumed. In a simplified format, the equation is as follows:

As an example case, let's assume a total V-8 engine piston displacement of 350 ci, giving us 43.75 ci/cylinder. If the section area of the intake runner is 3.0 square inches, we can plug these values into our little equation to calculate a corresponding torque peak at 6,048 rpm. At least this is where it should occur. Of course, this only addresses how the intake manifold will contribute to the engine's overall torque curve. If we observe this boost (from the intake manifold) occurring at a lower rpm, we could say the engine is "under-cammed," and if it appears at a higher rpm, the engine could be over-cammed.

So, one use for our equation is to evaluate how a particular intake manifold will influence overall torque, particularly at its mean flow velocity. If we'd like to select/design/modify an intake manifold's runner section area to boost torque at a desired point, the equation can be algebraically rearranged to solve for the required section area to read as follows.

In this case, let's assume we'd like an intake manifold torque boost at 5,800 rpm (for whatever reason, like gearing, track length, and so on) and need to know the section area associated with this engine speed. Inserting these values into our little equation tool computes an intake flow path area of 2.88 square inches.

As also previously discussed in this column, flow path length is a factor in how an intake or exhaust system contributes to overall torque output. The rule of thumb here is length affects how a given torque boost "rocks" about its peak rpm point. It's the section area that relates to the peak point since cross section size (flow rate) links directly to engine speed or piston displacement.

For example, given a fixed section area, the 240 feet/second mean flow velocity will occur sooner as piston displacement is increased. And, of course, the opposite is true if engine size decreases. (We've included a simplified illustration intended to help you visualize these relationships.)

So what about intake and exhaust flow paths of non-uniform cross section? Since we're attempting to stay with the hands-on approach to making these concepts a useful tool to engine builders, tuners, and parts manufacturers, we'll avoid how intake passage taper plays into the issue.

For the sake of simplicity, you can calculate the entry and exit section areas, average the two, and use that number for flow path section area in the equation. While it won't provide the most refined data, you may be surprised at how useful it can be.

On the exhaust side, when using headers with specific "steps" or sudden changes in section area, here's how you can view that subject. When what we'll call an exhaust "pulse" experiences a sudden change (increase or decrease) in section area, there will be a corresponding reaction in a reverse direction.

Again, simply stated, each section of primary pipe that differs from another will generate its own contribution to net torque from the exhaust system. And, as you might expect, the influence of each section's length on the whole is much like the intake side.

Now, where's the value in learning about and understanding this month's topic? If you're trying to evaluate an engine's performance, either on the dyno or track, knowing something about two major factors in overall torque output can help a range of topics, including on-track gearing, chassis set up, and driving technique. Certainly there are other engine components and conditions that affect net torque. But it's also a given that intake and exhaust systems have a major influence in where and how a racing engine operates in its intended speed range.

Can you use this "tool" to identify intake and exhaust system dimensions to optimize their application toward specific operational objectives?
Absolutely. Is it possible to size intake and exhaust systems to broaden a net torque curve by increasing the rpm range between their respective torque peaks? Again, absolutely.

Just remember that when you look at an overall torque curve (or data) that displays only one peak, that doesn't mean each system isn't contributing its separate part. I've been a part of tests that helped verify this by literally tuning an exhaust system well beyond a test engine's rpm range to more clearly define torque contribution by the intake system, dimensioned as described in the column you're reading. We then tuned the intake system beyond the available rpm range after re-dimensioning the exhaust system to evaluate its contribution. The peaks for each were remarkably close to the predicted rpm, based on the engine's piston displacement and intake/exhaust dimensions. So the idea works. And when you stop and think about it, it's a quick way to evaluate and match parts to either minimize mistakes, reduce parts investment, or both.